1. Field of the Invention
The present invention relates generally to aircraft flight controls, and more specifically, to actuated forebody strakes for yaw control at high angles of attack.
2. Description of the Related Art
Fighter aircraft engaged in aerial combat are required to make aggressive maneuvers, particularly during one-on-one combat with other fighter aircraft. These aggressive maneuvers frequently require deceleration to low speeds and high angles of attack (a).
A major factor which limits the high angle of attack effectiveness of current fighter aircraft is the degradation of rudder yaw control as angle of attack is increased. The level of yaw control provided by a conventional rudder decreases as the angle of attack is increased because the vertical tail becomes immersed in the low-energy stalled wake generated by the wing. This presents a problem since the level of yaw control required increases due to the higher yaw rates required to coordinate rolling maneuvers at the higher angles of attack. For current models of fighter aircraft, the result is that the amount of yaw control required exceeds the amount available beginning at an angle of attack substantially less than the angle of attack for maximum lift. As a result, the roll-rate of the aircraft is limited and thus leads to reduced maneuverability.
When high angles of attack are experienced, the forebody itself remains in undisturbed flow and creates a pair of powerful vortices.
The forebody portion of an aircraft has become an increasingly important aerodynamic factor in aircraft design. In most modern fighter aircraft, the moment arm from the forward-most point of the forebody to the center of gravity is equal to or greater than the moment arm of the vertical tail or tails. Therefore, the forebody is well suited for placement of control surfaces which can take advantage of the powerful vortex flow field and long moment arm associated with the forebody.
In one previous attempt to control the vortex generated by the forebody at high angles of attack, jet blowing over the forebody is used to influence the direction of the naturally occurring vortex asymmetry in order to effect yaw control. However, because of the non-linear characteristics of the natural vortex asymmetry, jet blowing has not been effective for controlling the level of yawing moment produced. Moreover, jet blowing, when effective, is limited to small sideslip angles due to the strong effect of sideslip on the vortex asymmetry.
An asymmetric nose strake attached to the long, slender forebodies of aircraft has been investigated as a means for generating yawing moments at high angles of attack. Neilhause, A.I. et al., "Status of Spend Research for Recent Airplane Designs" (NASA TR R-57, 1960); Chambers, J.R., et al., "Effects of a Pointed Nose on Spend Characteristics of a Fighter Airplane Model Including Correlation with Theoretical Calculations" (NASA TN D-592I, September 1970). In these prior investigations, an "asymmetric" strake is a fixed surface attached to the forebody at a radial location other than in the vertical symmetry plane. It was discovered from these prior investigations that a single strake attached to one side of an aircraft forebody is capable of producing consistently large yawing moments. However, the asymmetric nose strake was considered for generating large yawing moments to be used for spin recovery, and the possibility of modulating the level of yaw control for use as an active control device was not considered.
Aircraft forebodies which exhibit a substantial natural vortex asymmetry at high angles of attack would require a symmetric nose strake deployment to reduce the vortex asymmetry when no yawing moment is required. One such symmetric nose strake deployment is accomplished with a longitudinally hinged strake concept which forces symmetric flow separation when the strakes are deployed in a symmetric position. In the case of longitudinally hinged strakes, opposite-side strakes are deflected differentially to generate a vortex asymmetry to provide the desired yaw control. For longitudinally hinged strakes, a "conformal" strake is used when the strakes are required to be retracted at low angles of attack. In a conformal strake, the strake pivots longitudinally and conforms to the side of the forebody. Forebodies that do not produce a substantial natural vortex asymmetry would not require an initial symmetric strake deployment. In this case, a pair of conformal strakes are implemented such that either strake could be deployed depending on the direction of yaw control required.
The longitudinally hinged forebody strake has been shown to provide enhanced levels of yaw control at high angles of attack. However, the concept is limited by the fact that modern fighter aircraft normally have forebodies which exhibit complex curvatures and cross-sectional characteristics that vary along the length of the forebody. Thus, the design and fabrication of longitudinally hinged conformal forebody strakes would be technically difficult and expensive. Also, the radial location of the hinged strakes determines the angle of attack and sideslip ranges of effectiveness. Since the hinged strake concept provides for strakes that would be fixed at a predetermined radial location, the angle of attack and sideslip ranges of effectiveness would be inherently limited. Another problem is that for military aircraft having radar mounted in the forebody section, performance of the radar can be adversely affected by moving metallic parts placed ahead of the radar unit. Therefore, the longitudinally hinged forebody strake, which has moving metallic parts, including at least hinges and linkages, could reduce radar performance